1. Field of the Invention
The present relates to a method of manufacturing a semiconductor device and, more particularly, to an improvement in a technique of patterning a metal film, an insulating film, and the like.
2. Description of the Related Art
In a conventional process of manufacturing a semiconductor device, a metal film, e.g., a metal wiring layer is formed in the following manner. A conventional method of forming a metal wiring layer will be described below with reference to FIGS. 1A to 1G.
As shown in FIG. 1A, after an insulating film 52 is deposited on a semiconductor substrate 51, a lower wiring layer 53 is formed on the insulating film 52, and an inter-layer insulating layer 54 is deposited on the entire surface of the resultant structure.
A metal film 55 is formed on the inter-layer insulating layer 54, as shown in FIG. 1B. Thereafter, as shown in FIG. 1C, a photoresist (photosensitive resin layer) 56 is directly coated on the metal film 55.
Subsequently, as shown in FIG. 1D, the photoresist 56 is exposed with a desired pattern. The photoresist 56 is subjected to a development process to form a photoresist pattern 56.
As shown in FIG. 1F, the metal film 55 is selectively etched by the reactive ion etching (RIE) method using the photoresist pattern 56 as a mask.
Finally, as shown in FIG. 1G, the photoresist pattern 56 is removed to complete an upper wiring layer consisting of the metal film 55.
In a method of this type, however, the following problems are posed.
Light 57, which has passed through and reached the upper surface of the metal film 55 in the process of exposure, is reflected by the surface, and reflected light 58 enters the photoresist 56 again. As a result, abnormal exposure occurs in the photoresist 56. Especially, as shown in FIG. 1D, if the upper surface of the metal film 55 has large irregularities, the influence of the abnormal exposure is increased. As a result, as shown in FIG. 1E, the pattern transferred to the photoresist 56 is distorted as compared with the mask pattern. This distortion of the photoresist pattern 56 poses a serious problem when the metal film 55 is patterned by the RIE method using the photoresist pattern 56 as a mask.
That is, since part of the photoresist pattern 56 is omitted, as shown in FIG. 1E, the corresponding part of the patterned metal film 55 is also omitted, as shown in FIG. 1F. In the worst case of this phenomenon, the patterned metal film 55 is disconnected, resulting in a great deterioration in precision and reliability of patterning. In addition, the yield of products is decreased.
In order to solve the above problem, a method using a pigment-containing resist and a method of forming a titanium nitride film between a metal film and a photoresist have been proposed.
In the method using a pigment-containing resist, the distortion of a photoresist due to abnormal exposure cannot be satisfactorily suppressed, and hence the above problem cannot be solved. In addition, the focusing margin in the process of exposure is undesirably reduced.
In the method using a titanium nitride film, after a metal film is selectively etched, a titanium nitride film on the metal film is removed to prevent corrosion of the metal film and the like. However, since it is difficult to set a sufficiently high etching selectivity between a titanium nitride film and an insulating film, the insulating film is undesirably etched when the titanium nitride film is removed by etching. As a result, the reliability and yield of products are decreased.
In addition to the above-described methods, a method of forming a fine pattern by forming a carbon film between a film to be processed and a photoresist film has been proposed in Published Unexamined Japanese Patent Application No. 60-117723.
In the method, however, dimensional change occurs between a size of a carbon film and that of a photoresist because the carbon film has a thickness of as large as 180 nm or 110 nm. Assume that a carbon film having a thickness of e.g. 100 nm is to be patterned by etching, using a diode type plasma etching apparatus, under the following conditions: an oxygen flow rate of 100 SCCM, a pressure of 40 mTorr, and a power density of 2 W/cm.sup.2. In this case, a side wall portion of the carbon film is tapered at an angle of about 63.degree.. Therefore, if the carbon film has a thickness of 100 nm as described above, a size difference of 90 nm or more is caused between the bottom and top of the carbon film. In semiconductor integrated circuit, it is desired to decrease such a dimensional change.
Journal of Technical Disclosure No. 78-2427 (Japan Institute of Invention and Innoration) discloses an exposure is performed by intervening a carbon film between a wafer and a photoresist film, thus preventing the reflectance by the wafer. However, both the thinner carbon film and lower reflectivity are not realized yet.
In a conventional method, an insulating film such as an inter-layer insulating layer is patterned in the following manner. FIGS. 2A to 2G are sectional views showing the respective steps in the method.
As shown in FIG. 2A, an insulating film 62 is deposited on a semiconductor substrate 61, and a lower wiring layer 63 is formed on the insulating film 62.
As shown in FIG. 2B, an insulating film 64 as an inter-layer insulating layer is deposited on the entire surface of the resultant structure. Thereafter, a photoresist (photosensitive resin layer) 65 is directly coated on the insulating film 64, as shown in FIG. 2C.
Subsequently, as shown in FIG. 2D, the photoresist 65 is exposed with a desired pattern. Thereafter, as shown in FIG. 2E, the photoresist 65 is subjected to a development process to form a photoresist pattern 65.
As shown in FIG. 2F, the insulating film 64 is selectively etched by the reactive ion etching (RIE) method using the photoresist pattern 65 as a mask.
Finally, as shown in FIG. 2G, the photoresist 65 is removed to complete patterning of the inter-layer insulating layer 64.
In this method, the following problems are posed.
As shown in FIG. 2D, part of incident light 66, which has passed through the photoresist 65 in the process of exposure, further passes through the insulating film 64 to reach the upper surface of the lower wiring layer 63, and is reflected by the surface. Reflected light 67 enters the photoresist 65 again. Another incident light 66a passes through the insulating films 64 and 62 to reach the semiconductor substrate 61 and is reflected by its surface. Reflected light 68 then enters the photoresist 65 again. The photoresist 65 is exposed again with the reflected light 67 and the reflected light 68. In this case, since the optical characteristics of the lower wiring layer 63 are different from those of the semiconductor substrate 61, the intensity of the reflected light 67 is different from that of the reflected light 68. In addition, since the distance that the reflected light passes through the insulating film is different from the distance that the reflected light 68 passes through the insulating films, a phase difference occurs between the beams of reflected light when they reach the photoresist 65, and a difference in intensity occurs when they interfere with the incident light 66.
As a result, the photoresist 65 is very differently exposed depending on positions, and hence the mask pattern cannot be faithfully transferred to the photoresist 65.
FIGS. 3A to 3G are sectional views showing the respective steps of another conventional method of patterning an insulating film.
As shown in FIG. 3A, the upper surface of a semiconductor substrate 71 is selectively oxidized to form insulating oxide films 72. An insulating film 73 and a polycrystalline silicon film 74 are then formed on the entire upper surface of the substrate 71 on which the insulating oxide films 72 are formed.
After an insulating film 75 is deposited on the polycrystalline silicon film 74, as shown in FIG. 3B, a photoresist 76 is directly coated on the insulating film 75, as shown in FIG. 3C.
After the photoresist 76 is exposed with a desired pattern, as shown in FIG. 3D, the photoresist 76 is subjected to a development process to form a photoresist pattern 76.
Subsequently, as shown in FIG. 3F, the insulating film 75 is selectively etched by reactive ion etching (RIE) using the photoresist pattern 76 as a mask. Finally, as shown in FIG. 3G, the photoresist pattern 76 is removed to compete patterning of the insulating film 75.
In this method, however, the following problems are also posed.
As shown in FIG. 3D, incident light, which has passed through the photoresist 76 in the process of exposure, further passes through the insulating film 75 to reach the upper surface of the polycrystalline silicon film 74, and is reflected by the surface. Reflected light 78 then enters the photoresist 76 again. As a result, portions, of the photoresist 76, other than the portion corresponding to the mask pattern are exposed, thus causing abnormal exposure. Especially, as shown in FIG. 3D, if the upper surface of the polycrystalline silicon film 74 has large irregularities, the influence of the abnormal exposure is increased. As a result, the pattern transferred to the photoresist 76 is distorted as compared with the mask pattern.
If the thickness of the insulating film 75 varies at different positions, the phase of the reflected light 78, which has reached the photoresist 76, varies depending on the incident positions. Therefore, when the reflected light 78 interferes with the incident light 77, variations in intensity of light are caused, and the photoresist 76 varies in size at the respective positions.
The size differences due to the distortion of the photoresist 76 and those at the respective positions affect the polycrystalline silicon film 75. For this reason, the pattern formed on the insulating film 75 also suffers from size differences due to distortion and those at different positions. Such inconvenience leads to not only a deterioration in precision and reliability of patterning of an insulating film but also a decrease in yield of products.
In order to solve such problems, a method of patterning an insulating film by using a pigment-containing resist is proposed. In the method of using a pigment-containing resist, however, the distortion of a photoresist pattern due to abnormal exposure cannot be satisfactorily suppressed, and hence the above-described problems cannot be solved. In addition, the focusing margin in the process of exposure is undesirably reduced.
As described above, in the conventional process of patterning a metal film, when a photoresist is exposed, exposure light is reflected by the upper surface of a metal film, resulting in distorting the pattern transferred to the photoresist.
In the process of patterning an insulating film, since exposure light is reflected by the upper surface of a semiconductor substrate or that of a metal wiring layer, the pattern transferred to the photoresist is distorted. Since a film to be processed, such as a metal film or an insulating film, is patterned by using the distorted photoresist pattern as a mask, the precision and reliability of patterning are degraded, and the yield of products is decreased.